This invention relates to a method of forming fluoride glass fiber preforms for optical communication.
The application of fluoride glass fibers is expected in the field of optical communication, for example, the fiber optical amplifier operating at 1.3 .mu.m-band, the optical waveguide and the fiber laser because its light transmittance region is wide and the transmittance of infrared region therethrough is particularly high as compared with oxide glass fibers and plastic fibers. However, there are some drawbacks of fluoride glass in the preparation of optical fiber preforms. That is, fluoride glass tends to react with water in the atmosphere and to be crystallized.
There are some usual methods of forming fluoride glass fiber preforms. For example, JP-A-57-191240 discloses a build-in casting method. In this method, firstly, a fluoride glass melt for the cladding is poured into a mold which comprises a plurality of separable elongate portions. Then, an unsolidified central portion of the fluoride glass melt is allowed to flow out. After that, another fluoride glass melt for the core is poured into the central portion of the mold.
JP-A-58-125630 discloses a rotational casting method. In this method, firstly, a fluoride glass melt for the cladding is poured into a rotating mold. Then, an unsolidified central portion of the fluoride glass melt is allowed to flow out. After that, another fluoride glass melt for the core is poured into the central portion of the rotating mold. However, the above-mentioned two methods have the following drawbacks.
It is needed to repeat casting in a relatively short time. Bubbles and striae tend to be incorporated into the glass because it is necessary to pour a core glass melt into a central opening of a cladding glass tube. Furthermore, the cladding glass is reheated and thus tends to be crystallized by pouring a large amount of the core glass melt into the cladding glass tube which has started to solidify.
JP-A-3-183630 discloses an extrusion method in which a solidified core glass body and a solidified cladding glass body are mated with each other at their ground surfaces and the mated glass body is extruded.
JP-A-63-190739 discloses a rod-in-tube method. In this method, a solidified core glass rod is inserted into a solidified cladding glass tube so as to form a preform. After that, the preform is melted, and then the melted preform is drawn.
The above-mentioned extrusion method and rod-in-tube method have the following drawbacks.
In the methods, at first, the core glass and the cladding glass are separately prepared. Therefore, the incorporation of impurities into the glass tends to be suppressed. However, a high gain can not be obtained due to irregular interface between the core and the cladding. Furthermore, it is difficult to prepare a preform for a single-mode optical fiber and in particular to have a diametral ratio of cladding to core more than about 15:1.
JP-A-63-11535 discloses a suction method in which a cladding glass melt is poured into a mould which is formed at its lower end with a sink for the cladding glass melt, and then a core glass melt is poured on the cladding glass melt. During cooling of the cladding glass melt, a void space is produced at the center of the cladding glass body due to contraction of the cladding glass in the sink. Accordingly, the void space is filled with the core glass melt so that a preform having a core-cladding structure is prepared. However, this method has the following drawbacks.
The core diameter does not become constant because the central void space for the core is produced due to only contraction of the cladding glass in the sink. Furthermore, it is difficult to obtain a long preform.
JP-A-4-31333 discloses a method in which a preform made by the rod-in-tube method or the suction method is drawn so as to obtain a core having a uniform diameter, and then the outer surface of a cladding is ground so as to obtain the cladding having a uniform diameter. However, in this method, crystallization at the interface between the core and the cladding tends to occur, thereby lowering strength of the preform.